The Ship of Theseus and Personal Identity

The Ship of Theseus is a classical philosophical puzzle about personal identity. I’ll give the quick version. (Note: if you’re a professional philosopher, you will not be happy with the quick version. But you likely won’t be happy with much of anything I write here, anyway.)

Imagine you have a wooden ship in your backyard and decide that one day you want to take it apart piece by piece. You go about taking it apart, delicately removing each part as if you were going to use those same parts to rebuild it later (should you decide to do so) and storing it in your garage.

Now, here’s part of the problem. When you remove each piece, it still seems to be (intuitively) the same Ship. Removing one plank from the floor, for example, doesn’t seem to make it a different ship. However, if you keep up with the process, you will end up with all of the pieces of the Ship in your garage, but it will no longer be the Ship of Theseus, since, by hypothesization, you won’t have a ship – you’ll have a pile of wood that used to be a ship.

But at some point in your deconstruction, the Ship had to move from existence to non-existence, unless you want to say all the pieces in the garage is the Ship. At what discrete point did the Ship cease being the Ship?

A further wrinkle: suppose that, rather than just tearing the Ship apart, you decide to replace every wooden piece you removed with an aluminum piece of the exact same dimensions. So, when you start, you have a completely wooden Ship, but at the end, you have a completely aluminum Ship. But, at each discrete stage of time, you only have a ship that is one piece different than it was in the previous moment.

An even further problem: suppose that you decide to use the wooden planks you removed in the case above to build another Ship which is materially identical to the original ship. At the end of that project, you’ll have two Ships, one aluminum and one wooden, that each have a claim to being the Ship of Theseus. They can’t both be THE Ship of Theseus, but it could be true that they both could NOT be the Ship of Theseus, but the problem becomes, when was the Ship of Theseus destroyed?

A few options:

The Ship of Theseus is what it is by the individual parts that make it up.

In this case, the second you removed the first wooden plank from the Ship, it ceased to be. But the individual atoms in the wood are forever changing, with the result that the Ship is never itself.

The Ship of Theseus is what it is because of its structure.

In this case, the Ship remains the same ship throughout the change from wood to aluminum, so you have the seemingly contradictory result that the Ship of Theseus is both an aluminum and a wooden ship. Furthermore, when you have two ships (as in the last thought experiment), they both have identical structures, so you wind up with the result that both are THE Ship of Theseus – meaning that two discrete things are one numerical thing.

The Ship of Theseus is what it is because of its history.

In this case, the Ship remains the same because of its particular role in the history of the world. Parts come and go, but the actor remains the same. You’ll still wind up with the problem in the case of Theseus duplicates, because each share a relevant history with the “original” ship.

The Ship Represents Us

Now, the real problem is not at all about ships, but instead about that that makes us who we are. We know that parts of who we are changes from year to year, but we still think we’re the same people.

Is it because of our parts – i.e. the individual matter that makes us up? Breathe, and you’re no longer the same you.

Is it because of our structure? Lose a limb or cut your hair and you’re no longer the same person.

Is it because of our history in the world? Were you to be duplicated, you’d either have a existential twin, or you’d cease to be.

Is it because of our thoughts, feelings, and all the other stuff that goes on in our heads? Lose your memories, and you’re no longer you. Have a radical change of heart, and the person you were once before is gone.

Is it because of our souls? Souls could logically be duplicated and would run the same risks of the Ships. (Sidebar: soul talk, in general, is a way to understand what it is that is us through time. Our physical bodies come to pass and theories of personal identity that posit spiritual existence before and after physical existence have to have some device to allow for identity through the different phases of our existence.)

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Comments

A decidely unphilosophic answer for you: the blog is YOUR THOUGHTS IN PRINT so it’s the same blog. Or arguably it has to do with the URL, the name, or the design, in which case it’s changed. But I’ll stick with the former argument.

You raise such an interesting dilemma and then apply it to something that, well, I can’t see how you’re asking your readers to apply it. But nonethless, it is always a challenge to decide what defines us and whether our core is the same, whether it doesn’t matter if the core is the same, how much we’re allowed to veer from how we once acted in the past, etc. Anyhow, food for thought…

@ Jared: About the blog – that’s the interesting thing, for there are really compelling reasons to go either way with what makes this the same.

I’m really not asking my readers to apply anything – which is probably why you can’t see how I’m asking them to apply it. My (poorly executed) aim was just to give some idea of how I got to thinking about the Ship of Theseus – it wasn’t from what I’m working on in philosophy, but rather the mundane statements having to deal with my “new” blog.

Secondly, it’s a common philosophical tool to ask questions about things that aren’t really the issue, get an answer, then apply that to the core issue. If you ask direct questions, people often have pre-theoretical answers and won’t budge from those answers. That’s sometimes what makes philosophy so frustrating to people who don’t understand what we’re up to – for we ask questions in really oblique ways.

Your post reminds me of the dilemma I face in wanting to connect and reconnect with old friends from certain distinct stages of my life. It’s a paradox because we’ve grown in different directions at different rates. We want to see each other when we’re all back in our hometown for holidays, because we like the memory who we were together, then. But we really don’t connect the same anymore, because we’ve grown into substantially different versions of ourselves, and we have mixed feelings about finding out who we’ve each become — would our memories and our stories about ourselves and each other be less valid if it turns out we’re not who we thought we were? They remember details of the past as enduring character traits of mine, and I’ve completely forgotten or outgrown what I did or said then. I think it takes a lot of energy to keep friendships intact over time, distance and experience.

@ Kelly: You’ve touched on a very personal chord with me, as well. It’s very, very difficult for me to return to my home town and talk to old friends. Since the inevitable “what do you do” question comes up, it becomes very hard for us to relate to each other since our lifestyles are so very different. Yet I don’t want to not spend time with them just because we’ve gone separate ways. So we have awkward conversations about days gone by since we don’t have a lot of common points from the here and now to talk about. Hearts and thoughts, they fade… (Pearl Jam reference)

1) Theseus’ ship is completely replaced, timber by timber, over the course of its use – Even though the original components are replaced, it still functions as Theseus’ ship, therefore is the true ship. Even if some of the component parts differ somewhat from their originals (which must be the case, since “fresh” timbers replace “rotting” ones), the intent is to preserve the original ship as conceived by the builders, designers, and owner, namely the ship of Theseus. In this case, to be overly concerned with the materials is to fail to see the ship itself.

2) A new ship is build from the “old” components of the original ship – Again, context is key. The intent is not to build Theseus’s ship (which already exists and is owned by Theseus), but to build a new ship (not belonging to Theseus) entirely out of the components removed from Theseus’ ship. It is not intended to be Theseus’ ship, it is therefore a new ship. The line becomes grey if Theseus owns this “new” ship – at that point the dilemma becomes entirely Theseus’.

3) Theseus’ ship is now on display at a museum, and over time, thieves steal all the component parts of the ship (one at a time) and replace them with replicas – In this case, the component parts define the ship, as the intent of the museum is to display the ship that was given to them (in a sense it has stopped being Theseus’s ship, and is now the museum’s historic relic of Theseus’ ship). The theft/replacement of the component parts was not authorized by the owner of the ship (the museum); the thieves own a stolen ship, and the museum owns a forgery. The line here gets fuzzy if the museum replaces the component parts since a) the intent is to preserve the ship as it was given to them, and b) it stopped functioning as Theseus’ ship once Theseus stopped owning it. (I would argue that if the museum were to replace components of the original ship in this case, their action renders the ship un-original since the intent is to display/preserve the ship as it existed during the time-slice at which the museum acquired it. They would no longer be able to claim the “original” ship, but a “restored” ship. At which point replacing parts changes the “restored” ship into a “replica” ship, I’ll leave that for you to ponder.)

4) Theseus is dead, and a new ship is built entirely out of components replaced in the original ship – Similar to number 2, except that this ship is built after Theseus expired. For historical value, you could probably claim that this is a version of Theseus’ ship, but good luck finding all the parts, and proving they’re originals.

Great discussion, Matt, but the critical assumption is that identity is an issue of semantics and not one of metaphysics. Taking the metaphysical meaning as the root of identity, then your examples are about what’s known as the Ship of Theseus – in this case the epistemic problem is piggybacking on the semantic problem. Or, maybe it’s not piggybacking, and they’re simply about what we mean by using the words “Ship of Theseus.”

The American pragmatist in me wants to accept the semantic orientation to this problem since that’s what really matters in most of our discussions, but the (conceptual) metaphysical query still stands. ;p

Yes, semantics has a part to do with identity, but my intended point was identity is based in belief. Even though all the parts of Theseus’s ship were replaced, Theseus believes it is his ship, and I’m sure no one on his ship would dissent. The museum believes only the original components of the ship constitute the real ship, therefore if pieces were replaced one-by-one, in the museum’s belief (and probably in the patron’s as well), the ship essentially becomes a forgery, even though the design and the essence of the ship has not changed.

Consider these additional examples:

Over the course of a year the average human consumes 1.5 tons of water, food, and oxygen. Assuming he has not gained 3000 pounds, this stuff (most of it anyway) has to be assimilated into the body, pushing out “old” stuff. It is estimated 98% of the body’s molecules have been replaced over a year. Over a year-and-a-half, over 99% of the body’s molecules have been replaced. So, do you believe yourself to be a completely different person than you were a year-and-a-half ago? Probably not – you believe you are the same because there there is continuity in your “self”, your ideas, your identity. It is your belief.

Contrarily, if I could collect all those molecules from your body that were expunged over that year-and-a-half, and reassemble them as they were in your body, would that new creation be you? I’m sure you wouldn’t think so. But maybe I do; maybe I believe this because I believe I have captured the “essence” of what is you. At that point our beliefs are at odds. Once our beliefs are at odds, the only thing left to distinguish between our beliefs is semantics. This is essentially what I was facetiously trying to convey in example number 2, above.

Then consider the “Washington’s axe” example. A museum owns Washington’s axe, but over the years, the handle has rotted so it was replaced. The head then rusted, and then was replaced. Is it still Washington’s axe? It is still an axe, the concept, design, and intent were preserved, but would you believe that it was still Washington’s axe? My belief tells me no, Washington never touched any of it’s component parts. It is a replica. The “essence” was lost.

There are three examples. The first argues that identity has little to do with component parts. The last argues that component parts are identity. The middle example, the best example, argues that identity is based upon both/neither. All show identity is based on perception, on belief. And when beliefs are at odds (like in the second example), semantics are all that’s left to sort things out.

One final example to ponder: The Gold Temple Pavilion in Kyoto, Japan was built in the 14th century. Over the course of 7 centuries, it has been destroyed and rebuilt several times. But to the Japanese, there is no paradox – they believe the temple that stands today is the temple.
“The idea of the building, the intention of it, its design, are all immutable and are the essence of the building. The intention of the original builders is what survives. The wood of which the design is constructed decays and is replaced when necessary. To be overly concerned with the original materials, which are merely sentimental souvenirs of the past, is to fail to see the living building itself.”
””Douglas Adams

Though Mr. Adams didn’t at first believe that the rebuilt building was the building, once he talked to the Japanese, he got it. It is all about belief.

(Note: this example was appropriated from Wikipedia, where there is an excellent discourse on the Ship of Theseus.)

Interesting question. I have been thinking about mitosis and the ship as well.

So going to this:

” We know that parts of who we are changes from year to year, but we still think we’re the same people.”

Our neurons do not get replicated, which in my opinion, the brain cells are the defining aspect of our personal identity. So because the neurons remain constant for the most part, our identity is maintained. Once the neurons malfunction, then we lose portions of our identity. Now if they can every replicate neurons, I guess that is when the ship will really come into play.

The chemicals are only different in the sense that it is a newer source producing them, but the chemical type is still the same, because it still follows the same blue print instruction set. So the neurons, which remain unchanged during the mitosis process, still process the data the same.

If there are malfunctions during cell replication then I can see how identity can be affected because the chemical type itself may be different and can impact the nervous system in ways that induces change.

Maybe I am not following, I guess after you elaborate I can truly assess your position.

While I’m not attempting to “solve” the riddle of Theseus’s ship, two insights did occur to me (and no doubt to others).

Firstly, the rebuilt ship is a but like “that river you can’t step twice in” because when you do the original water that flowed past you has moved on, and new water is flowing past a second time. But both that and this seem to rely on sloppy or mistaken definitions of the subject we’re considering. A river for example isn’t the sum of the water within its banks, but rather a geographical expression of where water has flowed, is flowing, and likely to flow. The course of rivers change over time but we still (mostly) tend to refer to them by the same names. Likewise the rebuilt ship is a temporal event created with the original building of it, and continuing on over a period of time, even though it may not have any of the original parts, which like the water in the river, have moved on.

Mind you the missing element is the perception of the river, or the ship, by those who interact with it. The “identity” is something created by and for humans and has no meaning without them.

Secondly, it seemed to me that the second ship was more of a “clone” of the original, regardless of whether or not it used the original parts, because it required an act of deliberate recreation. Instead of genes and DNA, we have someone with a plan of what to build. Of course we have clones in nature – twins, and no one asks if either twin is the same (or do they?).

I never really understood this paradox, since the paradox itself seems non existent to me. This is why I also not really get the metaphor.

For me the The ship of T is the ship of T as long as it is T’s ship. The ship you build with discarded material would be a ship build with discarded material of the ship of T. You can add that somewhere in time all the material used in the ‘replica’ ship were part of the ship under the command of T.

Is it T’s ship? no, assuming he is not the owner.

I understand that it is a metaphor, but I am not getting it.

In humans, as a determinist, my being is determined by the parts I am made out of. Though brain cells arent really replaced, the atoms are. So even though I would argue that I am the same now as I will be tomorrow, if one wants to be specific or talk semantics, I would agree with the proposition I am different now then tomorrow. I am but the causality of probability, it has little to do with “parts” imo. and yet it has everything to do with parts because I do believe that I am an aware biological computer. If that computer changes, then it changes. If you could clone me exactly how I was ten years ago, then you would have a version of me from ten years ago.

I just dont see the paradox. If its only about defining “who am I” then its a bit of a weak one.

The challenge is with the strict definition of identity; two separate things can’t be one and the same thing. In your last case, there are completing claims on which separate entity is ‘Tom’. Both are, but can’t be.

For me the The ship of T is the ship of T as long as it is T’s ship.

If you mean ‘T’s ship’ to be possessive, then we have a problem: change ownership and you change identity. My car becomes a different thing when I give it to you. What happens when it’s in between ownership?

If you mean “T’s ship” to be something like “it’s itself as long as it’s itself”, then you’ve said something true but not really useful for the conversation. “My car is my car as long as it’s my car.” Such a statement doesn’t get at what we care about: what makes it ‘it.’

A metaphysics that includes souls or some similar construct is where most of the rub is, to be honest. A mechanistic or nihilistic view of the human condition tends to be fairly matter of fact: change the parts, you change the person. Done. What’s the problem?

Sure, two Toms is awkward, but it’s only awkward due to minor annoyances like legality and morality. For instance, it would be heinous to punish your current timeslice for something a past timeslice did just as much as it would be wrong to punish me for something you did. Your prior timeslice is an ontologically different person from you in the same way that I am a different person from you. That person may be more related, but a sibling is also more related and still doesn’t deserve to pay for your actions.

This is exactly why various spiritual systems make sense. The Buddhist theory that that what we call a person is a collection of various things. These include objective and tangible features such as the body and subtle features such as the mind. However, beyond these features, there remains an awareness that does not require the world. For instance, in the state of the deepest sleep where the body and mind have little relevance to the personality, the awareness exists. There may be remembrance of it cause to remember it is a feature of the mind (memory). But the awareness exists and comes to the fore with the needs of the body and mind happening backstage.

The same awareness exists even in a wakeful and a ‘normal’ state but is kept in the background. Every spiritual aspirant seeks to make this awareness the primary source of identity. The state so ‘achieved’ being referred to as enlightenment. Once there, all paradoxes disappear.

I believe that buddhist philosophy has something addition to add to the apparent paradox of Theseus’ ship. It is this:

“Theseus’ ship” has no inherent objective existence independent of the designation that we human beings choose ro give it. In other words, the pieces of wood are so arranged that we choose to think of them and refer to them as “a ship.” However, all that ever really exists is some pieces of wood arranged in a certain way. “Theseus’ ship” is merely a name or a designation which a particular group of humans choose to bestow on a particular arrangement of pieces of wood at a particular time.

Hence, according to buddhist philosphy Theseus’ ship never existed in the first place in any real sense independent of the minds of those who were contemplating it. As the ship never really existed in the first place then all the subsequent arguments regarding what is (or is not) the “real” Theseus’ ship become redundant as the ship was never “real” in the first place. The entire apparent “paradox” is thereby exposed as stemming from the initial false assumption that compounded things in the material world have any “real” existence.

We give a collection of elements a “name” and then we believe that “name” to represent something “real” and “unchanging:” But we are deluding ourselves because nothing in the material world is permanent and unchanging. Permanence is an illusion. The “ship” is an illusion. All that ever existed is a collection of ever changing elements that we choose to designate “ship” or “chair” or “person” at any particular point in time.

I believe we should all take our hats off to Buddha because it appears he explained a solution to this so-called “paradox” over 2500 years ago.

I believe we should all take our hats off to Buddha because it appears he explained a solution to this so-called “paradox” over 2500 years ago.

And yet …

Following this answer to its logical conclusion, nothing exists. Again, the Ship of Theseus isn’t about “material” material objects as much as it is about the nature of existence itself. Whether those elements are material elements or non-material elements is irrelevant.

The proposed solution may be valid, but the conclusion that nothing exists appears to be unsound. While Descartes’ classic maxim – cogito ergo sum – can be deconstructed in the same way such that the “I” that is doing the thinking really doesn’t exist, there’s a “somethingness” that remains that does exist. We could apply the paradox to that, as well, with the same fork.

“And yet following this answer to its logical conclusion nothing exists.”

Whilst certain thinkers of both Eastern and Western traditions have held that nothing exists (and thereby opened a whole new can of worms) I do not believe this is necessarily the “logical conclusion” of Buddha’s solution when we consider the rest of his analysis of reality. This is because Buddha stressed impermanence and also distinguished between what he called “conventional truths” and what he called “ultimate truths.”

Ultimate truths are things that are real regardless of our beliefs about them. Conventional truths are things that are only true because we designate them as being true. Atoms are ultimately true/real (though impermanent.) Ships, chairs and iphones whilst real in a conventional sense are, as compounded things, not ultimately real. We have given a collection of atoms a name and believe that collection of atoms to be something other than what it really is. It is still simply a collection of atoms arranged in a certain way even after we have named it. The act of naming a collection of atoms arranged in a certain way as “Theseus’ ship” does not magically transform those atoms into something other than a collection of atoms arranged in a certain way. It is still merely a collection of atoms arranged in a certain way that we choose to designate at a particular time as “Theseus ship.” Buddha said “in space there is no East or West. Men make these distinctions in their minds and then believe them to be true.” The same could be said of “Theseus’s ship.”

Impermanence means that whilst matter and energy do exist (an ultimate truth) they are impermanent. Matter and energy are constantly changing, in a state of flux combining and splitting over time. Thus whilst nothing has a permanent existence it is not a “logical conclusion” that nothing exists at all at any particular point in time.

What we refer to as “Theseus’ ship” is solid. We can sail in it. In that sense it is “real.” However it is real only in the conventional sense. Ultimately what we choose to call “Theseus’ ship” is an impermanent collection of atoms that we have chosen to designate as “Theseus’ ship.” When we understand reality in this way the apparent “paradox” disappears.

You’re addressing the semantic and epistemological layers of the dilemma quite well AND, again, the horn of the dilemma is not really about “what we call a collection of things” or “what we perceive/know about a collection of things,” but about what makes that thing, that thing.

To say “we conventionally know that collection of stuff to be X” is only to say that we’ve named a collection of stuff. To claim there’s nothing unique about that collection of stuff that makes it that stuff rather than other something else is an option to take, but that just follows the classical horns of the Ship of Theseus.

Invoking ultimate truths actually only shuffles the ontological question from material stuff to non-material stuff, such a) what makes it ultimate and b) what is the nature of existence for an ultimate truth? Did the ultimate truth “exist” before things existed, or is it contingent on the existence of something, in which case, in what sense is it ultimate? Is an ultimate truth true of all possible worlds, i.e. what’s the ontological status and essence of ultimate truths?

The Ship of Theseus is one of those classic paradoxes because of logical entanglements – i.e. the bump in the rug phenomenon. You can fix one part of the problem only by shifting the problem somewhere else. There are many ideologies that would happily accept that selves are nothing but impermanent collections of atoms. Dilemma “solved”, save for the entanglements.